Monocrystalline materials sometimes offer a high degree of uniformity in terms of their physical properties as well as a high degree of repeatability and efficiency when compared to polycrystals of the same material. When monocrystals with these properties are discovered, they are highly prized as replacements for polycrystalline materials, wherever such materials are used. Moreover, monocrystalline materials may also be sought out for applications for which polycrystalline material would never be considered. For example, in optical applications, polycrystals will provide a diffuse reflection of incident light whereas monocrystals would yield specular reflection.
However, while monocrystals are desirable, the technical problems associated with their fabrication are significant. The methods employed to produce polycrystals are inadequate for the production of larger single crystals. Finding an appropriate method of growing monocrystals can be completely unpredictable. For example, if a system is useful for the production of a binary metal oxide crystal, it does not follow that the same production protocol would be useful for producing a ternary crystal, even when two of the three metals involved were successfully grown in the binary system. In fact, even if a flux system was useful in growing one specific binary metal crystal, there is no way to predict how that same flux system would behave if one of the metals was exchanged for another.
Moreover, it is nearly impossible to predict how a growth/flux system will behave. The same amounts of starting materials could yield crystals of different structure and composition when produced using different types of flux.
Finally, while the properties of monocrystals are often superior to their polycrystalline counterparts, there is no way to predict the relative behavior of monocrystals based on polycrystals. In fact, there are times when monocrystals exhibit inferior properties when compared to polycrystals. For example, some devices such as ceramic ferrite memory cores, depend on the random orientation of many small grains. Single crystals do not work. Even when the properties are superior, there is no way to predict the degree of improvement.
In accordance with the present invention, and indeed generally, monocrystals can be differentiated from polycrystals based on a number of factors. Monocrystals are sized and shaped such that they can be used individually in the production of sensors, probes and the like. Polycrystalline materials are those made up of a composite of many individual crystals. Many ceramic materials are polycrystalline in nature as are many rocks and fabricated metals. The size of these polycrystals are usually small with equivalent diameters in many materials varying from a few micrometers to about 100 micrometers. Much larger crystallites are possible. However, because of physical properties such as, for example, packing density and other issues common in the ceramics field, it is usually advantageous to insure that the individual crystals do not get too large and are of as uniform a size and composition as possible. In the context of the present invention, however, when referring to a single crystal, preferably, an isolated crystal which is large enough to manipulate i.e. sliced or shaped, is envisioned. This allows one to take advantage of the properties of same with respect to the individual orientation of the crystal. In preferred embodiment, individual crystals are about 100 microns in size or greater and more preferably, at least about a millimeter in size or greater, along one edge. Certainly, crystals of a centimeter on a single edge would be considered rather large.
There are other significant differences as well, most of which stem directly from the fact that monocrystals are, as their name implies, singular, whereas polycrystals involve the interaction of at least two crystals and suffer from charge carrier scattering at their grain boundaries, where modifications to the electric conductivity likely take place. In fact, transport processes such as resistivity and dielectric properties of the crystal differ considerably from those of ceramic samples. Polycrystalline materials can be thought of as a composite material made up of two or more distinct individual components. Just as the polycrystalline materials are composites of the individual crystals, so too are their properties. Polycrystals have voids and often certain other stoichiometry and phases. These features each have an effect on the overall properties of the material and any device or sensor made using them. For example, the resistivity (.rho.) of these semiconducting monocrystals is somewhat less than that of the corresponding semiconducting polycrystals of that same material because of the presence of grain boundaries. The permitivity (.epsilon.) of the polycrystalline material is also affected thereby. Monocrystals, which do not suffer from such composite properties will not exhibit such a strong dispersion in their impedance-frequency characteristics.
Moreover, since no two groups of polycrystals can be exactly the same, i.e. same number of crystals of identical size, orientation, stoichiometry and composite properties, the response of one sensor made with one group of polycrystals may vary with respect to other such sensors. Polycrystals may also be problematic because they may absorb water, particularly in the voids between crystals. When such material is exposed to variations in humidity, "aging" or a lack of reproducibility of properties over temperature may be accelerated in comparison to comparable monocrystals. Moreover, the size of the voids between individual polycrystals may change with time and exposure to the elements and in response to external electric fields. Again, the thermal and electrical properties of the resulting material may therefore change over time. Monocrystals do not suffer from these same aging limitations.
Finally, with regard to certain materials and in particular cubic spinel crystals, there may be magnetic ordering effects at lower temperatures in monocrystals. But with polycrystals, exposure to a magnetic field may cause distortion of the individual crystals. This would result in a change in the grain boundaries and the size and shape of any voids and lead to hysteretic effects (non-reproducibility). The composite properties of the material would change accordingly. Certainly, the magnetic, thermal and electric properties of monocrystals can be more accurately measured and more repeatedly relied upon than polycrystalline materials.
Cubic spinel crystals such as the crystals of the present invention, provide isotropic properties when compared to crystals of other geometric configurations. For example, transport processes, such as electric conductivity, are isotropic.
Polycrystals of nickel-manganese-oxides and manganese-cobalt oxides have been reported by a number of investigators and, in particular D. G. Wickham, et al. .sup.(1-3), and their properties have been studied and documented. Dr. Wickham's work, along with his collaborators, illustrated and characterized many of the advantageous properties of these polycrystalline cubic spinels. The use of a boron/bismuth flux for the growth of nickel manganite single crystals is disclosed in H. Makram, "Growth of Nickel Manganite Single Crystals," Journal of Crystal Growth, (1967), 325-366. The manufacture of single crystals of NiMn.sub.2 O.sub.4 and their use as thermistors is disclosed in Rosen et al., U.S. application Ser. No. 485,851, filed on Jun. 7, 1995, which has received a notice of allowability.
Polycrystals of nickel-cobalt-manganese were reported by, for example, L. V. Azaroff, see Z. Kristallogr., volume 112, pages 33-43, (1959). However, monocrystals of these materials are believed to be unknown. Moreover, despite the success the inventors achieved throughout the growth of nickel- manganese oxide monocrystals, there was no way to predict how the introduction of an additional variable to the system (namely cobalt) would affect the ability to produce monocrystals. Particularly, there was no way to predict that the result would be the uniform production of a single phase nickel-cobalt-manganese oxide having a cubic-spinel structure.